The semiconductor industry has a requirement for the production of semiconductor devices that is most often met by fabrication of arrays of many devices on a single wafer. The semiconductor devices often require doping to very fine tolerances to achieve desired characteristics. Such doping may be performed using an ion implanter that comprises an ion source to generate ions corresponding to, or containing, the required dopant. Optics then form the ions into a focussed ion beam that is incident upon the wafer. Control of the ion beam (e.g. beam current, ion content, energy, size, scanning, etc.) is of paramount importance as this determines the dopant concentration in the wafer and also the depth of implant, thereby determining the conductive properties of the devices.
Typical dopants include boron, phosphorus, arsenic, aluminium, antimony and indium. Ions of these dopants are often produced in the ion source by obtaining a gas containing the required dopant, allowing the gas to enter an arc chamber where an arc discharge ionises the gas to form a plasma. An extraction electrode is used to extract ions from the arc chamber through an aperture provided therein. Further electrodes are used to form an ion beam that is directed at the wafer to be implanted. Generally, the ion beam passes through a mass-analysing magnet that selects only ions with the desired mass/charge ratio: put another way, the mass-analysing magnet effectively rejects unwanted ions that are inevitably produced in the arc chamber/plasma or otherwise generated.
The gas supplied to the arc chamber may be obtained in a variety of ways. One method is to heat the elemental form of the dopant (invariably a solid) in an oven. The vapour so produced is allowed to pass into the arc chamber. However, many of the dopants are metals with low vapour pressures meaning the oven must be operated at high temperatures to produce the required vapour.
Alternatively, compounds containing the dopant of interest may be heated in an oven. U.S. Pat. No. 2002/0029746 discloses heating indium fluoride to achieve indium doping. Adjusting the beam current requires an adjustment of the temperature of the oven and control is therefore limited by the thermal response time of the oven (as much as 30 minutes). Moreover, control is unpredictable because the true temperature of the contents of the oven cannot be known precisely. The low vapour pressure of indium fluoride poses another problem in the condensation of the vapour so produced. Thus, transport of the vapour becomes difficult.
These methods of producing a dopant gas species pose a problem because they show great sensitivity to variations in temperature, i.e. a graph showing how their vapour pressure varies with temperature is particularly steep around the operating temperature of the ovens. As a result, there is a burden in that fine control of the oven temperature is necessary. Typically, the oven must be controlled to better than 1° C.
Another approach to producing the required dopant in gaseous form is to pass a gas over the dopant (or a compound thereof) such that the two react to form the required gas that drifts into the arc chamber. This technique has been known for quite some time, both in ion implantation and in other fields. For example, Sidenius and Stilbreid in E.M. Separations with High Efficiency of Microgramme Qualities (E.M. Separation of Radioactive Isotopes, Proceedings of the International Symposium, Vienna, May 1960, Springer-Verlag, pp 244–249) discloses passing carbon tetrachloride over heated rare-earth oxides to form gaseous chlorides of the rare earth. Halides such as carbon tetrachloride are often used because of their high vapour pressure. More recently, U.S. Pat. No. 6,001,172 discloses passing a variety of gases such as fluorides (NF3, ClF3, BF3 and fluorine itself) over heated indium or antimony to produce fluorides of indium or antimony that are then ionised.